This is invention relates to a four cycle engine and more particularly to an improved cooling, valve operating, and air intake structure for the cylinder head of such an engine.
The use of overhead valves is well known in four cycle engine to improve their efficiency and performance. In fact, it is now the practice to employ multiple intake and exhaust valves for the purpose of performance and efficiency improvement. In one popular valve and port arrangement for such engines, there are provided a greater number of intake valves than exhaust valves. In a specific form, there are employed three intake valves and two exhaust valves. Where there are a greater number of one type of valves than the other, it is normally the practice to use larger head diameters on the lesser number of valves and a greater lift therefore. However, this gives rise to certain difficulties.
When one set of the valves has a larger diameter and greater lift, stiffer springs must be employed for closing the valves and longer springs must be employed so as to permit the greater degree of lift. Since it is normally the practice to use symmetrical camshaft locations for operating the different valves, this means that both camshafts become raised relative to the cylinder head and the overall height of the engine becomes large. On the other hand, if there is an attempt to make the engine more compact by compressing the height of the cylinder head on the side where the lesser number of valves are employed, this gives rise to a different problem. When this is done, then the cooling jacket around the lesser number of valves can be restricted. This is a particular problem since the lesser number of valves are generally the exhaust valves and a greater degree of cooling is required for this side of the cylinder head.
It is, therefore, a principal object to this invention to provide an improved cylinder head arrangement that accommodates different number of intake and exhaust valves and still provides adequate cooling without excessive cylinder head height.
It is a further object to this invention to provide an improved cylinder head arrangement for a multi-valve engine wherein a greater height cooling jacket may be provided on one side of the engine than the other without having the overall engine height become excessively large.
It is a further object to this invention to provide a valve operating structure for the cylinder head of an engine which permits the use of different numbers of intake and exhaust valves and tailors the valve spring and tappet operating mechanism for each valve to suit its specific needs without providing unduly large cylinder head or one that does not have adequate cooling.
It is generally the practice, as afore suggested, to employ separate intake and exhaust camshafts for the valves of an overhead valve, multiple valve engine. Where, however, the engine employs different number of intake and exhaust valves, the camshafts may not be positioned symmetrically. This can give rise to problems in properly driving the camshafts and orienting the overall engine construction.
It is, therefore, a still further object to this invention to provide an improved camshaft drive arrangement for an internal combustion engine.
Where multiple valves are employed, the placement of the valves and their valve actuators also presents some design problems. When overhead camshafts are employed, normally thimble tappets are interposed between the camshafts and the valve stems for their actuation. However, when multiple valves are employed then the supporting structure for these tappets in the cylinder head assembly can present some problems. That is, if a number of bores are positioned closely adjacent to each other, the intervening area between the bores may be weak. This is a particular problem when light alloy cylinder head material is employed.
It is, therefore, a still further object to this invention to provide an improved cylinder head arrangement that facilitates the use of multiple tappets positioned close to each other and yet supported by a relatively strong structure.
In one form of cylinder head construction particularly useful with multiple valve engines, the cylinder head is actually divided into two components. These are a lower head portion and an upper cam carrier portion. The cam carrier portion carries the camshafts and also the tappets that operate the valves. Where such an arrangement is employed, however, this gives rise to the difficulties in porting the cylinder head. That is, the ports must, with prior art constructions, be disposed so that they only extended through the lower cylinder head portion. This substantially compromises the porting system.
It is, therefore, a still further object to this invention to provide an improved porting arrangement for a multiple valve, two piece cylinder head construction for an internal combustion engine.